A new planning tool for analyzing power systems in detail, including the long-run technical, economic and environmental consequences of policy or investment interventions, is now available for download by the public, without charge, at E4ST.com. The Engineering, Economic and Environmental Electricity Simulation Tool and its components are described here, and its accuracy is assessed through predictions of actual LMPs in the Eastern Interconnection. The usefulness of the tool is illustrated by an evaluation of the commercial feasibility of a proposed electric transmission line connecting Hydro Quebec to New York City, the Champlain-Hudson Power Express.